1. Field of the Invention
This invention relates to an NBR (acrylonitrile-butadiene rubber) composition, and more particularly, to an NBR composition having a high resistance to adherence to metals.
2. Description of the Prior Art
Sealing members made of rubber, such as O-rings, packings and gaskets, are often used in intimate contact with metal surfaces, such as of shafts and covers, and the adherence of rubber to the metal surfaces creates trouble when the sealing member has to be changed. For example, the rubber adhering to a shaft, or cover makes its removal difficult, or is difficult to remove completely from its surface. A change of parts made under such circumstances results in the formation of an incomplete seal.
A number of methods have been proposed for preventing such adherence of rubber, but none of them has been found satisfactory from the standpoint of effect, productivity, or cost, as stated below:
(1) Blooming or Bleeding:
A lubricant, or plasticizer having low compatibility with rubber is, for example, added to rubber so as to bloom or bleed from the rubber to form a layer for preventing it from contacting a metal surface. This method is, however, not very effective, since the material which has bloomed, or bled out of rubber is likely to flow away, depending on the object to be sealed. Moreover, the rubber is likely to have its physical properties lowered by the addition of the material having low compatibility with it.
(2) Coating:
A molded and vulcanized product of rubber has its surface coated with a film of a solid lubricant, such as graphite or molybdenum disulfide, or of a silicone coating agent. Such a film, however, peels off easily. The coating material is likely to form foreign matter in the object to be sealed. Moreover, its application calls for an additional job and an additional cost of manufacture.
(3) Chemical Surface Treatment:
A molded and vulcanized product of rubber has its surface hardened by treatment with a halogen treating solution containing chlorine or fluorine, or an acid, or alkali to acquire resistance to adherence to a metal surface. The hardened surface of rubber, however, makes it resinous, and likely to crack by deformation, and thereby give an incomplete seal. Moreover, the treatment calls for an additional job and an additional cost of manufacture.